I've been trying to figure this out myself lately. You can check out my thread "Filtering a 480".
I'm pretty sure I've decided to drill the back of the tank and run lines through a bag or cartridge filter, then through 1 or 2 bead filters, then back to the tank.
I don't want to worry about evaporation through a sump, or needing it filled by my wife when I'm out of town.
800 gallons more or less? I would say a large Pond cannister filter with an FX5 or two for polishing. Most anyone building a tank that size does a sump. Seems a little small for sand pool filter but it would work too! look for used pool equiptment.
I carnt do a stump for this build its wooden framed with glass on the one side, so need something like a pool pump, but i thought koi pumps wasent good enough for tropical or is it myth of what people say ?
Dav.